A ring shear apparatus was used to investigate the development of clay smear along faults in sand–clay sequences. Experiments were performed, using six different clay types, different stress conditions (σn=6–500 kPa) and different amounts of clay (4 and 12.5%). The development of clay smear seems to depend on the competence contrast between the clay and the surrounding sand. Clay, when it is less competent than sand, behaves in a ductile manner, resulting in the development of clay smears along the fault. Clay which is more competent than sand behaves in a brittle manner, resulting in the formation of angular fragments. Whether the clay is more or less competent than the sand is dependent on the stress conditions, the initial porosity of the sand and the mechanical properties of the clay. The results suggest that stress conditions allowing the sample to contract will result in the formation of fluid flow barriers, whereas dilation results in the formation of conduits.